The present invention relates to digital transmission systems. More specifically, the present invention relates to echo cancellation in transmission over digital modems.
In a typical analog modem implementation, a modem is connected through a hybrid circuit to the telephone network local loop. A local loop connects the local hybrid circuit to the local central office. The trunk network connects the local central office to the remote central office, which is then in turn connected to the remote hybrid circuit through a local loop at the remote end.
A hybrid circuit connects the modem or telephone two wire circuit to the local loop four wire circuit. Each circuit carries a signal in both direction, transmit and receive. A typical telephone network employs a hybrid converter to connect the unidirectional four wire link from the PSTN to the local two wire loop. The four wire circuit has one wire pair for the signal in each direction. The two wire pair must duplex the signal to carry both transmit and receive signals on a single wire pair. Part of the transmitted signal from the modem or telephone can be reflected back to the transmitter by the hybrid circuit, resulting in the speaker hearing his or her own speech delayed as an echo. This is referred to as near end echo.
A far end echo can also occur when the transmit signal passes through the PSTN and is reflected back by the hybrid at the far end of the PSTN.
In the typical analog modem implementation, FIG. 1, the modem 1 is connected through the hybrid circuit 2 to the telephone network local loop 3. The hybrid circuitry 2, as well as D/A and A/D converters 4 and 5, are selected to provide maximum linearity and the minimum possible noise floor, so that the overall modem performance, including the echo canceler, is maximized. With good implementation, the near end echo, which presents the major impairment in an analog modem, is canceled to the level of background noise floor. The far end echo, on the other hand, is passed through the PSTN 6 and to some degree non-linearly distorted due to non-linearities present in the network. However, the level of far end echo is attenuated due to local loop attenuation.
Unlike analog modems, digital modems, FIG. 2, are connected to the PSTN through digital T1 or E1 lines. The near end echo is typically nonexistent, because there is no local four to two wire conversion. On the other hand, the far end echo is more emphasized, because it is not attenuated by a local loop, since no local loop is present.
The signal which returns from the far end will pass through a PCM to linear converter before being combined with the echo cancellation signal. The echo cancellation signal and the signal received from the far end are both linear.
A modem echo canceler is typically implemented as a linear adaptive filter, which is used to adapt to the echo path. In theory, it is known that the filter convergence of a linear adaptive filter will be fastest when the input signal is uncorrelated. In an analog modem implementation, a typical near end echo loop is highly linear, and the input data to the echo canceler is uncorrelated (data-driven echo canceler), to provide for the optimal convergence properties.
However, in the digital modem case, the far end echo path, once it has returned to the near end, has passed through two PCM to linear and linear to PCM conversions, which present a significant source of non-linearities. One of those conversions is performed within the digital modem, and the other one is performed in the remote central office as part of the D/A or A/D conversion in the hybrid circuitry.
The typical PSTN digital modem connection is subject to far end echo which cannot be completely canceled using conventional approaches, i.e., a linear adaptive filter. In essence, the echo canceler synthesizes the far end echo which is then subtracted from the composite signal of the far end signal and echo. There are significant non-linearities present due to double conversions with the result that an echo canceler according to the prior art does not model the true echo path well and is limited to about 33 dB of rejection.
In order to improve the reduction of echo in a digital modem, the near end sample is taken after linear to PCM conversion and passed through a PCM to linear conversion before generation of the echo cancellation signal and recombination with the far end signal. The echo cancellation will more closely match any far end echo due to the introduction of conversion distortions more closely resembling those created by the conversion of the signal during transmission through the network.
The conversions experienced by the signal path are nonlinear in nature due to the xcexc-law or A-law conversions, and they present a major problem to a linear adaptive filter used in echo cancelers, since the filter is not able to cancel them. When linear signal samples are fed directly to an echo canceler, the amount of echo rejection is limited to about 33 dB. The present invention can improve echo rejection to about 36 dB.
In digital modem connections there is a need to improve echo rejection beyond 33 dB. The current invention, provides a transmitter modem configuration which introduces a linear to PCM conversion and a PCM to linear conversion in a conventional echo canceler line with a resultant improvement in the amount of echo rejection.
A modem-based echo cancellation system and method for increasing the amount of echo rejection in a digital modem. This increase is achieved by passing modem signals from the linear to PCM conversion, through digital delay line and a PCM to linear conversion before passing to the echo canceler. The signal provided to the echo canceler is thus converted from linear and reverted back to linear, partially matching the conversion distortion of the far end echo path, minimizing far end echo path non-linearity with the echo cancellation signal.
The present invention improves echo rejection in a PSTN digital modem connection. The present invention minimizes far end echo path non-linearity in an echo canceler line in a PSTN digital modem connection. The present invention reduces memory requirements for the echo canceler delay line by after linear to PCM conversion storing samples at low computational cost in a PSTN digital modem connection.
These and other objects are achieved by providing a digital modem-based echo cancellation system which feeds distorted transmit modem samples output from a linear to PCM conversion to an echo canceler line which incorporates an initial delay. The signal is next passed through a PCM to linear conversion. A linear adaptive filter then produces the echo cancellation signal. By modeling one linear to PCM conversion of the far end, in this way, the far end echo path non-linearity is minimized and rejection is improved.